The invention relates to a method of reducing the permeability of underground strata during secondary recovery of oil.
When an oil well is first drilled, oil will often flow from the well under the natural pressure existing in oil-bearing strata. When this natural pressure becomes insufficient, further quantities of oil may be recovered from the well by a mechanical pump. However, it is well known to those skilled in the art that even when no more oil can be recovered from the well simply by mechnical pumping, large quantities of oil often still remain in the oil-bearing stratum, especially if the oil is a heavy, viscous type of crude oil. To recover at least part of this residual oil, which will not flow naturally to the bottom of an oil well penetrating the oil-bearing stratum, so-called "secondary" recovery techniques have been developed. In such secondary recovery techniques, a flooding, liquid (which may be, for example, water, brine, an aqueous solution of a polymer, an aqueous solution of a surfactant or a hydrocarbon fluid) is pumped down an injector well. The flooding liquid flows from the injector well through the oil-bearing strata and forces at least part of the residual oil into a producer well. In some cases, it is advantageous to inject steam down the injector well since the resultant heating of the oil-bearing strata reduced the viscosity of the oil present therein and assists flow of the oil to the producer well.
Unfortunately, the various zones within an oil-bearing strata often differ greatly in fluid permeability. Often it is found that there are fractures within the oil-bearing strata; these fractures may be natural or may occur because of the fracturing normally effected near the bottom of a producer well in order to assist oil flow thereinto during the first phase of the recovery. Alternatively, no actual fractures may be present within the oil-bearing stratum but there may be zones of, for example, loosely packed sand of very high-permeability. In some cases even if no high-permeability zones are originally present in the oil-bearing stratum, such zones may be created by the action of the flooding liquid sweeping through the oil-bearing stratum. Where such high-permeability zones are present, almost all the flow of flooding fluid takes place along these zones with the result that, after a short period of flooding, almost all the liquid recovered from the producer well comprises mainly flooding fluid with only a small proportion of oil, while significant amounts of displaceable oil in zones of lower permeability within the oil-bearing strata are not recovered. Thus, the presence of such zones of high-permeability greatly decreases the efficiency of the secondary recovery process.
In order to overcome the aforementioned problems caused by the highpermeability zones in the oil-bearing stratum, it is known to inject into the stratum solutions which at least partially plug the high-permeability zones, thereby greatly decreasing the permeability of these zones, so that flooding fluid injected thereafter is forced to traverse other zones in the oil-bearing stratum, thus leading to increased oil recovery. The liquids used to plug the high-permeability zones are usually injected via the injector well, but may also be injected via the producer well if necessary. For example, U.S. Pat. No. 3,396,790, issued Aug. 13, 1968 to Eaton, proposes a method of plugging high-permeability zones in which water is first injected into a well at a high rate, then a viscous solution comprising sodium silicate, polyacrylamide and water is injected. After the injection of the viscous solution, water is again injected at a high rate and under high pressure, followed by injection of a less viscous solution containing ferrous sulfate and water. By carefully controlling the pressure and injection rates of the viscous and ferrous sulfate solutions, the two solutions react together to form plugs in the highpermeability zones.
U.S. Pat. No. 3,749,172, issued July 31, 1973 to Hessert et al, proposes a similar procedure for plugging high-permeability zones, but in which the plugging solution contains a polymeric gel.
U.S. Pat. No. 3,882,938, issued May 13, 1975 Bernard. describes a plugging technique involving the injection into the oil-bearing stratum of one or more aqueous solutions of reagents that react within the oil-bearing stratum to form a plugging material. One specific solution described is an aqueous solution of sodium silicate and a gelling agent such as an acid, an ammonium salt, a lower aldehyde, a polyvalent metal salt or an alkali metal aluminate.
U.S. Pat. No. 3,897,827, issued Aug. 5, 1985 to Felber et al, describes a gel forming solution consisting of a dichromate activator and a lignosulfonate solution containing an alkali metal or alkaline earth metal halide.
U.S. Pat. No. 3,583,486, issued June 8, 1971 to Stratton, describes a plugging solution containing an ethoxylated condensation product of a phenol and formaldehyde.
U.S. Pat. No. 4,212,747, issued July 15, 1980 to Swanson, proposes as a plugging solution a shear thickening polymer composition containing a high-molecular weight polyalkylene oxide polymer with a phenol/aldehyde resin, the composition being alkaline.
U.S. Pat. No. 4,246,124, issued Jan. 20, 1981 to Swanson, describes an aqueous plugging solution containing a water-dispersible polymer, an aldehyde and a phenolic compound, which may either by a simple phenol or a tannin such as quebracho or sulfomethylated quebracho.
To be effective in the wide variety of situations encountered during the secondary recovery of oil, any composition intended for plugging zones of high fluid permeability within the oil-bearing stratum must meet numerous requirements. The plugging solution must be sufficiently heat stable to plug the high-permeability zones at the temperatures of 60.degree. C. or more often encountered in oil-bearing strata. In addition, since brine is present within many oil-bearing strata, the plugging solution must be able to gel in the presence of brine and the formed gel must not deteriorate during prolonged exposure to brine. Since the high-permeability zones to be plugged are often still wet with oil, the plugging solution must be able to gel in the presence of residual oil especially in the presence of oil-wet sandstone often encountered in oil-bearing strata, and the gel must be stable in the presence of such oil. The form gel must also be resistant to all conventional flooding liquids, some of which may be used at elevated temperatures, and to steam injected to recover viscous oil from the oil-bearing stratum; this steam may be heated to temperatures of at least 290.degree. C. In order that the plugging solution may be pumped down the well and a considerable distance into the high-permeability zone, the solution should have low viscosity when first made up and should retain this low viscosity for the period (which may be several hours) necessary to pump the solution down a deep well and a substantial distance into the high-permeability zone. Once is position in the zone, the plugging solution should gel rapidly to a gel having a high mechanical strength. It is very desirable that the plugging solution be of a type which permits the operator, by varying the relative amounts of the various components in the plugging solution, to vary the time lag before gelling of the solution begins. Since in some cases it may be desirable to produce only a reduction in permeability of the high-permeability zones, it is desirable that the operator be able to control the composition of the plugging solution in such a manner as to allow only partial plugging of the high-permeability zones, thereby leaving some residual permeability therein. Furthermore, since it may sometimes be difficult to adjust the permeability of the zones to precisely the right degree immediately, it is desirable that it be possible to increase the permeability of the zones containing the plugging solution after the plugging solution has gelled. Finally, since the plugging solution will often be subjected to considerable shear forces as it is pumped into the porous high fluid-permeability zones, it is important that the plugging solution not be affected by such shear forces.
No prior art plugging solution meets all these exacting requirements. Many prior art solutions produce gels of insufficient mechanical strength or will not gel properly in the presence of brine or residual oil. Moreover, many prior art plugging solutions are so viscous that it is difficult to pump them with sufficient speed and they are susceptible at least partial gelling before they have penetrated the high-permeability zones. In most cases, it is difficult to control the time lag before gelling of the plugging solutions occurs, so that gelling may occur before the plugging solution has penetrated beyond the bottom of the well, which may involve the expensive process of sinking a new well into the oil-bearing stratum. Finally, as shown below, prior art solutions based upon high-molecular weight polymers are susceptible to physical degradation by shear stresses, so that the shear stresses encountered during pumping of these plugging solutions into porous strata will greatly reduce the strength of the gel finally produced.
It will therefore be seen that there is a need for a method of plugging high-permeability zone in oil-bearing strata which fulfills all of the above-mentioned requirements, and the instant invention provides such a plugging method.